(Adapted from the applicant's abstract): The brain is the most frequent site of crippling and incurable human disease, and malignant primary brain tumors alone are more common than Hodgkin's disease, and cause more deaths than cancer of the bladder or kidney, leukemia, or melanoma. Conventional therapy for malignant brain tumors is ineffective and incapacitating, and represents the most expensive medical therapy per quality- adjusted life-year saved currently provided in the U.S. At the investigators institution, direct injection of (131)I-labeled, operationally-specific, monoclonal antibodies (MAbs) into brain tumor resection cavities delivers extremely high radiation doses to tumor cells around the resection cavity and has produced promising results in Phase II clinical trials. However, these MAbs diffuse only short distances beyond the cavity. Therefore, most of the radiation extending beyond the cavity is not specifically targeted to tumor cells and the radiation dose delivered beyond the cavity declines exponentially from the cavity interface. As a result, tumor cells that are known to infiltrate the brain for significant distances beyond the cavity are subopitimally treated and lethal tumors always recur within 2cm of the radiated resection cavity. Continuous microinfusion is a promising technique that allows homogeneous delivery of even large molecular weight molecules at high concentrations throughout large areas of the brain. Although this technique may enhance the delivery of (131)I-labeled MAbs and other therapeutic agents to diffusely infiltrating malignant brain tumors and reduce recurrence rates, the parameters that govern this technique and its limitations have not been defined. One of the major goals of this proposal is to define these parameters. In addition, this proposal is designed to investigate whether targeted radiotherapy might be improved through the use of human chimeric MAbs with increased biostability and the use of high linear energy transfer radioisotopes, such as (211)At, with greater relative biological effectiveness. The hypothesis to be tested in this proposal is that continuous microinfusion will widely deliver operationally tumor-specific MAbs conjugated to (131)I or the alpha-emitter (211)At such that they will be specific and potent therapeutic agents against malignant brain tumors with major reductions in toxicity to normal brain over conventional whole brain radiotherapies.